Definition of line
What is meant by "line" is the sequence of actions of a hand. A hand can be played in a multitude of ways. The flop can be check-call, check-raise, bet-raise-call, etc. When we play, we try to get an idea of our line ahead of time. If a player has 3bet AA preflop and gets a call, he may say to himself, "My line is going to be bet flop, bet turn and bet river. A single hand can have multiple "lines". Simply checking a flop instead of betting it will change the line.
Valuate its lines
If you're talking poker with friends, you can tell them about a hand you played that you want advice on. You can tell them how you played the hand and ask, "How do you like my line here? Your friends can then suggest alternative lines. The line should be seen as just one of many ways to play a hand. The right line to take will depend on several factors. For example, if you have a very strong hand and you are against a bluffing opponent who likes to bet when you pass, it is better to have a passive line than an aggressive one.